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"First in time, first in right" guides how water is divvied up within states but doesn't always apply across borders. Wyoming and Montana instead rely on the 1950 Yellowstone River Compact, which governs how the states share four Yellowstone tributaries -- the Powder, Tongue, Bighorn and Clarks Fork -- in times of scarcity. It guarantees both states continued access to any rights issued before 1950. To satisfy later rights, the states are promised fractions of each river's remaining flow.